Computer Based Meteor Search - a New Dimension in Video
Meteor Observation

Sirko Molau, Mirko Nitschke

The abilities and power of video meteor systems have recently been discussed in detail by
several authors [1,2,3].
The number of observers using this method to obtain high-quality
meteor data is increasing. They extend the results of meteor photography to fainter magnitudes
and smaller meteor showers. However, three main problems have to be solved before video
work can become as usual and frequently performed as photographic or even visual observations:
We need

high quality and cheap video meteor cameras,

a computer based meteor search system and

an effective measurement and analysis software for the captured meteors.

Meteor Cameras

Until recently, the main problem developing suitable video systems for meteor astronomy was
the price. Thus, most currently operated cameras are individual developments and stand-alone
systems. Basically all of them consist of an image intensifiers coupled to some type of optics
and an CCD imaging sensor.
However, especially in the last few years image intensifiers have become less expensive. Even
second generation devices (MCPs), which show much better characteristics for our purposes,
can now be purchased at reasonable prices well below $500, which makes the operation of
more amateur owned video systems only a question of time.
Recently, we have designed and built a video meteor camera series (Figure 1). The six
cameras consist of 0,75/50mm ultra-fast lenses, second hand MCP image intensifiers and simple
CCD video modules for recording the MCP's output screen. The systems are able to record
stars down to magnitude 8.5 and have an apparent field of view of 20 degrees in diameter.
Their design is very compact (approx. 100x100x300 mm) and robust. This makes them
especially suitable for field operation which is often the case in meteor astronomy.
To our knowledge, this is the first camera series with several cameras having the same structure
and therefore the same recording properties. This will be an considerable advantage when
combining data from different stations. In addition it was proven, that meteor cameras can
nowadays be built at prices below $800. The optical bench like mechanic design is open for
future hardware extensions. It can easily be modified and adapted to other components.

Figure 1 - the camera series before it's distribution among German meteor observers during the
annual AKM meeting in April 1996

Automated Meteor Search

One of the currently most difficult problems is the search for meteors. So far there is no system,
that is able to automatically detect meteors on video tapes in real time. Until now all tapes
have been manually inspected by the different observer teams and only first attempts of automation
were to be found in the literature [3]. It is obvious that effective large scale video
observation depends on the availability of automatic search systems, since the efficiency of
manual inspections is limited. There is no sense in developing large camera series and doing
regular observations if the video tapes cannot be analysed in reasonable times.
So far, the limited computing speed of available PCs together with noise problems of first
generation image intensifiers made the automated search infeasible. However, with the development
of new micro processor generations, price reductions for frame grabber cards in the
multimedia era and the availability of essentially noise-free MCPs, on-line image processing has
finally become possible.
In the last few months we developed and tested a prototype for an meteor recognition system
which proved to work fine with the hardware currently available. The principle of the search
algorithm was proposed earlier in [3] and is described in Figure 2:

Figure 2 - meteor search algorithm

A frame grabber digitises video frames at a rate of approximately 10 frames per second and
subtracts successive images from one another. As the result only changes in the image (i.e.
appearing and disappearing meteors) and noise remain visible. All persistent (i.e. stars) or slow
moving (i.e. satellites) objects will disappear and do not influence the following steps.
To reduce the noise, the resolution of the image is lowered by a factor of four in both axis. So
every new pixel is averaged from 16 raw pixels and the noise is reduced by 75%.
The next step involves a mask which is subtracted from the low resolution difference image.
This mask accounts for different sensitivity and noise within the intensifiers field of view. It is
dynamically generated from the average or maximum noise of the last n video frames (n = 10^2...10^3).
By subtracting the mask from the difference image, constant probabilities for meteor
detection in the entire field of view independent of camera properties and sky conditions are
ensured.
Finally, the procedure looks for longish objects in the resulting image. That is, for every pixel
the maximum sum of five neighbours aligned in different directions is calculated as shown in
Figure 3. This sum has to be bigger than a certain threshold to be counted as a meteor. The
threshold itself is similar to the mask dynamically computed from the average or maximum
noise of the last n meteor-free frames, multiplied by a detection rate factor r.
The r-factor tunes the sensitivity of the detection algorithm: Being only slightly bigger than 1,
it makes the procedure very sensitive to detect even faint meteors. However, the number of
misidentifications (i.e. the algorithms identifies noise as a meteor) increases dramatically. On
the other hand there will be almost no misidentification with higher r-values, but some of the
fainter meteor will also be missed.
If there is no pixel sum exceeding the threshold, no meteor is detected. If such pixel sums exist,
the computer counts them and stores the position of the brightest spot together with its time of
appearance. If the number of pixel sums exceeding the threshold becomes too big, something
'strange' happened (a change of the cameras field of view, for example). In this case the procedure
restarts calculating the noise mask, since general observing conditions may have
changed.
The described algorithm was implemented in Borland PASCAL including inline assembler
routines for all time critical sections. It drives an 512x512 pixel AT-bus frame grabber card
with 256 grey levels. The program was tested on a 486/DX2 66MHz and a Pentium 90 MHz
machine respectively, using the recordings from the alpha Monocerotid outburst in 1995. This
shower was recorded with the prototype of the new cameras series, thus, a video system using
an MCP intensifier. That is why the main problem we faced when we analysed MOVIE's video
tapes in 1993 [3], the strong electron noise of first generation intensifiers, did not occur.
In order to achieve an appropriate number of inspected video frames per second, the search
program needs to run four times, each time inspecting another part of the field of view (Figure 4
). Every frame contains approximately 90.000 pixel.

Figure 3 - for each pixel, 8 sums of five neighbours are calculated to find longish objects

Figure 4 - the search program runs four times inspecting different parts of the field of view

With this technique, the program was able to analysis every third non-interlaced video frame
(8.3 frames/s) running at the 486 PC, and every second frame (12.5 frames/s) when started at
the Pentium machine, respectively. The 30 minute test section of the Amo tape contained 28
meteors, 25 of which had been found by visual inspection. The computer detected 20 meteors
during the four necessary test runs achieving a detection rate of almost 75%. The r-factor was
set to 2.5 leading to only one misidentification. This low number is as important as the ratio of
detected to recorded meteors. Normally the meteor activity is much smaller than on the test
tape. The number of misidentifications should therefore not exceed 5 per hour to make the
search procedure effective.
It can be concluded, that automatic meteor detection is possible with today's computer technology.
The remaining 25% of undetected meteors occurred either in the small corners of the
field of view remaining uninspected, or they were just too faint. In both cases they are not
suited for further analysis anyway, since their positions is inaccurate due to the proximity to the
border, or they would be lost in the noise.
It turned out, that the CPU speed is not anymore the main problem. The real bottleneck is the
transfer speed from the frame grabber card to the computer's main memory, i.e. the bus system.
It is important to have a frame grabber that allows the transfer of raw images to the main
memory: Currently more common MPEG-compression boards are not suitable for meteor
detection.
The suggested inspection rate of 10 frames per second is appropriate for meteor detection.
Since even faint video meteors last in average at least 0.2 seconds, all of them will be visible on
at least one inspected video frame. It would be even critical to further increase the frame rate,
since especially slow moving meteors near a shower radiant would be missed due to their almost
stationary appearance.
We expect, that the detection rate and the number of test runs can still be improved. On the
one hand, the data transfer speed within the PC can at least be doubled using PCI frame
grabber cards. This implies an reduction down to two or even one test run necessary for each
video tape. In addition, there are still improvements in the back end of the algorithm thinkable.
They do not significantly increase the computing time due to the reduced size of the image, but
might still improve the detection rate.
Currently, the software has prototype status to study the properties and abilities of the
suggested algorithm. In the future it is planned to do a market analysis. We intend to find a
cheap frame grabber that matches the needs of video meteor observers, and to implement the
search program for that hardware.

Computer Based Measurement and Analysis

The problem of measuring video meteors was the first to be solved by several video observers.
Almost every team developed its own analysis software for different hardware and different
computer generations [4,5,6].
Recently, Marc de Lignie has expanded his AstroRecord
measurement program to video observations. The program is a hardware independent solution.
It requires Video for Windows AVI animation files as data input and does all the necessary
calculations to obtain the meteor's data. The software was introduced at the last IMC and
received much attention.
We suggest to decide for one software package for each of the described problems
(AstroRecord for the meteor measurement, for example) to avoid that all the programming
work is done again and again. This strategy ensures, that all meteor data are obtained with the
same procedure allowing us to directly compare results of different observers. In addition,
there has only one program to be maintained and improved with further progress in meteor
science and computer hardware.

Future Prospects

From the current point of view, the analogue recording of the sky using a VCR and video
tapes seems to be an appropriate solution. Real time image compression hardware, which
allows the storage of several hours of video signal on a computer's hard disc, as well as digital
video are currently under development. It is to expect, that it will take some more years until
the prices for such components have reached a suitable level for amateurs. Due to digital
broadcasting and the multimedia age as such there exists a mass market for such equipment, so
we would like to claim that as the next generation of video systems.
There have been discussions whether or not CCD cameras could be used to transfer the information
directly from the imaging sensor to the computer. This would make the optics and
electronics of the video camera as well as the frame grabber card redundant. There would be
no signal conversion from the CCD output via gain control to analogue video signals and back
to digital computer images. Hence, the noise and the computing overhead should reduce
significantly. However, currently in amateurs astronomy available CCD cameras serve other
purposes and do not reach the frame rate needed for meteor observation. This leads automatically
to longer integration times, and two of the main advantages of video systems are lost: the
ability to directly record the development of a meteor and the measurement of its velocity
without a shutter. In addition, integrated video cameras belong to the mass market and are
therefore relatively cheap. Stand-alone CCD cameras, however, are only used in some small
areas and will always be more expensive.

Conclusions

With the availability of cheap meteor cameras on the one hand, and the possibility to automate
the meteor search on the other hand, two remaining problems for the large scale usage of video
systems in meteor astronomy have been solved. It is expected, that a complete hardware and
software solution will be available by the end of the year, which makes extensive scientific
studies based on amateur video recordings possible. To achieve that aim it is essential, that
every video observers does not only record the sky, but also inspects and measures his own
video tapes. This will be possible due to smaller hardware costs and centrally available analysis
software.

Acknowledgements

We wish to thank Felix Bettonvil from the NVWS, who found those amazing ultra-fast lenses
for our meteor camera series at an optical flee market in the Netherlands. So far we didn't even
know, that such fast lenses do exist. We would also like to thank Detlef Koschny, who helped
purchasing the image intensifiers and contributed to the mechanic components of the cameras.